Pumping of fluids is specified in many processes. Often the most efficient way to control pump operation is using a frequency converter for driving the motor of the pump at a certain speed. Especially if the flow rate of the fluid is changed in a process, the use of a frequency converter provides the most economic control method for the pumping process.
Pump operating point location (flow rate Qv, head H) can be estimated by utilizing a frequency converter's torque and rotational speed estimates for an electric motor (Test and nest, respectively), pump characteristic curves given by the pump manufacturer, and affinity laws. Thus, the actual flow of the pumped fluid does not have to be measured for obtaining information on the operating point.
FIG. 1 illustrates an example of characteristic curves of a pump in accordance with a known embodiment. In particular, FIG. 1 shows an example of QH and QT curves and a method referred to as QT calculation. The principle of QT calculation is presented in U.S. Pat. No. 6,918,307 B2 in more detail. The pump manufacturer usually gives two curves: flow rate to head curve and flow rate to power curve. These curves can be used to produce a flow rate to torque curve which can be used to estimate the flow rate produced by the pump with the torque and rotational speed estimates of a frequency converter.
FIG. 1 shows the operating point estimation with the pump characteristic curves and the estimates of the frequency converter on rotational speed and torque. The flow rate Q produced by the pump can be determined from the lower curve when the pump shaft torque T is known. Then the head H produced by the pump can be read from the upper curve using the determined flow rate. Since the pump manufacturers usually give the characteristic curves only at one (e.g., nominal) rotational speed of the pump, the curves have to be changed to present rotational speed, as in the example of FIG. 1. This transformation of the curves can be carried out using affinity laws, as will be described later.
The accuracy of the estimation method is highly dependable on the accuracy of the pump characteristic curves. This factor limits the applicability of the QT calculation method for the estimation of the pump operating point location.
Currently, the QT calculation is based solely on the characteristic curves provided by the manufacturer, which can cause erroneous results, if the curves are not validated at all. Usually the torque consumed by the pump has a static or linear difference (bias) compared with the manufacturer's QT curve. For example, this difference can be caused by an improper assembly of the pump, manufacturing tolerances of the pump, pumped liquid, decreased efficiency due to the wearing of the pump, and characteristic curve measurement inaccuracies.
Document US 2007/0212210 A1 discloses a method of improving the accuracy of QT calculation. In this method the pump is operated against a closed discharge valve at several rotational speeds to determine the QT curve inaccuracy. However, when the pump is driven against a closed discharge valve, it does not produce flow. Instead it is in the harmful operating point, in which the pump does not normally operate. Hence, the closed valve test may specify interruption of the process, since pumps can be driven to provide flow in the process. This means that the prior art method is based on the use of a harmful and abnormal operating point of the pump.